Can somebody please explain to me why the answer to is ±2.5 and not just 2.5 and why the answer to is 2.66 and not ±2.66 ?

Hopefully you understand where my confusion is... it's about ± and when it should and shouldn't be used.

Substitute the negtaive solutions into the equation. Do they work? Why ....? What happens when a negative number gets raised to an even power ....? Would the same thing happen if you raised a negtaive number to an odd power ....?